Method of forming absorption enhancement structure for image sensor

ABSTRACT

In some embodiments, the present disclosure relates to a method of forming an absorption enhancement structure for an integrated chip image sensor that reduces crystalline defects resulting from the formation of the absorption enhancement structure. The method may be performed by forming a patterned masking layer over a first side of a substrate. A dry etching process is performed on the first side of the substrate according to the patterned masking layer to define a plurality of intermediate protrusions arranged along the first side of the substrate within a periodic pattern. A wet etching process is performed on the plurality of intermediate protrusions to form a plurality of protrusions. One or more absorption enhancement layers are formed over and between the plurality of protrusions. The wet etching process removes a damaged region of the intermediate protrusions that can negatively impact performance of the absorption enhancement structure.

BACKGROUND

Integrated circuits (IC) with image sensors are used in a wide range ofmodern day electronic devices, such as cameras and cell phones, forexample. In recent years, complementary metal-oxide semiconductor (CMOS)image sensors have begun to see widespread use, largely replacingcharge-coupled devices (CCD) image sensors. Compared to CCD imagesensors, CMOS image sensors are increasingly favored due to low powerconsumption, a small size, fast data processing, a direct output ofdata, and low manufacturing cost. Some types of CMOS image sensorsinclude front-side illuminated (FSI) image sensors and back-sideilluminated (BSI) image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some embodiments of animage sensor integrated chip comprising an absorption enhancementstructure configured to improve a quantum efficiency of an image sensorwithin the integrated chip.

FIGS. 2A-2C illustrate some additional embodiments of an image sensorintegrated chip comprising an absorption enhancement structure.

FIG. 3 illustrates a cross-sectional view of some embodiments of aback-side CMOS image sensor (BSI-CIS) comprising an absorptionenhancement structure.

FIGS. 4-8 illustrate cross-sectional views of some embodiments of amethod of forming an absorption enhancement structure for an imagesensor integrated chip.

FIG. 9 illustrates a flow diagram of some embodiments of a method offorming an absorption enhancement structure for an image sensorintegrated chip.

FIGS. 10-18 illustrate cross-sectional views of some embodiments of amethod of forming a BSI-CIS comprising an absorption enhancementstructure.

FIG. 19 illustrates a flow diagram of some embodiments of a method offorming a BSI-CIS comprising an absorption enhancement structure.

FIGS. 20-26 illustrate cross-sectional views of some embodiments of analternative method of forming a BSI-CIS comprising an absorptionenhancement structure.

FIG. 27 illustrates a flow diagram of some embodiments of an alternativemethod of forming a BSI-CIS comprising an absorption enhancementstructure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

CMOS image sensors (CIS) typically comprise an array of pixel regions,which respectively have an image sensing element arranged within asemiconductor substrate. Color filters are arranged over the imagesensing elements and are configured to filter incident light provided todifferent image sensing elements within the CIS. Upon receiving light,the image sensing elements are configured to generate electric signalscorresponding to the received light. The electric signals from the imagesensing elements can be processed by a signal processing unit todetermine an image captured by the CIS.

Quantum efficiency (QE) is a ratio of the number of photons thatcontribute to an electric signal generated by an image sensing elementwithin a pixel region to the number of photons incident on the pixelregion. It has been appreciated that the QE of a CIS can be improvedwith on-chip absorption enhancement structures. For example, anabsorption enhancement structure comprising protrusions arranged along asurface of a substrate can increase absorption by decreasing thereflection of incident radiation. Such protrusions are typically formedusing a dry etching process. However, the dry etching process used toform the protrusions can result in plasma damage along outer edges ofthe protrusions. The plasma damage can lead to defects (e.g.,interstitials) in a crystalline structure of the substrate, which cancause an increase in dark current and/or white pixel number. Theincrease in dark current and/or white pixel number causes charges toaccumulate within an image sensing element when light is not impingenton the image sensing element, thereby becoming a major source of noisethat can degrade image quality of a CIS.

The present disclosure relates to a method of forming an absorptionenhancement structure on a substrate having an image sensing element,and an associated apparatus. The method reduces crystalline defectsresulting from plasma damage sustained by the substrate duringfabrication of the absorption enhancement structure. In someembodiments, the method may be performed by forming a patterned maskinglayer over a first side of a substrate. A dry etching process isperformed on the first side of the substrate according to the patternedmasking layer to define a plurality of intermediate protrusions arrangedalong the first side of the substrate within a periodic pattern. A wetetching process is performed on the plurality of intermediateprotrusions to form a plurality of protrusions. One or more absorptionenhancement layers are subsequently formed over and between theplurality of protrusions. The wet etching process removes a region ofthe plurality of intermediate protrusions that is damaged by the dryetching process and that can negatively impact performance of an imagesensing element within the substrate.

FIG. 1 illustrates a cross-sectional view of some embodiments of animage sensor integrated chip 100 comprising an absorption enhancementstructure configured to improve a quantum efficiency (QE) of an imagesensing element within the integrated chip.

The image sensor integrated chip 100 comprises a substrate 102 having aplurality of pixel regions 103 a-103 b. The plurality of pixel regions103 a-103 b respectively comprise an image sensing element 104configured to convert incident radiation (e.g., photons) into anelectric signal (i.e., to generate electron-hole pairs from the incidentradiation). In some embodiments, the image sensing element 104 maycomprise a photodiode. Isolation structures 106 (e.g., shallow trenchisolation structures, deep trench isolation structures, isolationimplants, etc.) may be arranged within the substrate 102 at locationsbetween adjacent ones of the plurality of pixel regions 103 a-103 b.

A plurality of transistor devices 108 are arranged along a first side102 a of the substrate 102. A back-end-of-the-line (BEOL) metallizationstack is also arranged along the first side 102 a of the substrate 102.The BEOL metallization stack comprises a dielectric structure 110surrounding a plurality of conductive interconnect layers 112. Thedielectric structure 110 comprises a plurality of stacked inter-leveldielectric (ILD) layers. The plurality of conductive interconnect layers112 comprise alternating layers of conductive vias and conductive wires,which are electrically coupled to the plurality of transistor devices108.

A second side 102 b of the substrate 102 comprises a non-planar surfacedefining a plurality of protrusions 114 arranged in a periodic pattern.The plurality of protrusions 114 are laterally separated from oneanother by recesses 116 within the second side 102 b of the substrate102. In some embodiments, the plurality of protrusions 114 may compriseangled sidewalls 115 respectively comprising a first segment 115 ahaving a first sidewall angle ⊖₁ (or slope) and a second segment 115 boverlying the first segment 115 a and having a second sidewall angle ⊖₂(or slope) that is larger than the first sidewall angle ⊖₁ (or slope).In some embodiments, the first sidewall angle ⊖₁ may be in a range ofbetween approximately 45° and approximately 55°. In some embodiments,the second sidewall angle ⊖₂ may be in a range of between approximately75° and approximately 90°.

One or more absorption enhancement layers 118 are arranged over theplurality of protrusions 114 and within the recesses 116. One of the oneor more absorption enhancement layers 118 contact the substrate 102along the non-planar surface to define an absorption enhancementstructure 120 having an interface with a topography that increasesabsorption of radiation by the substrate 102 (e.g., by reducing areflection of radiation from the non-planar surface). Increasingabsorption of radiation by the substrate 102 increases a quantumefficiency (QE) of the image sensing element 104, and thereby improvesperformance of the image sensor integrated chip 100.

FIGS. 2A-2C illustrate some additional embodiments of an image sensorintegrated chip comprising an absorption enhancement structure. FIG. 2Aillustrates a cross-sectional view 200 of an image sensor integratedchip comprising an absorption enhancement structure. FIG. 2B illustratesa top-view 224 of the image sensor integrated chip of FIG. 2A and FIG.2C illustrates a three-dimensional view 228 of the image sensorintegrated chip of FIG. 2A.

As shown in cross-sectional view 200, the image sensor integrated chipcomprises a substrate 202 having a plurality of protrusions 204 arrangedin a periodic pattern within a pixel region 103 comprising an imagesensing element 104. The plurality of protrusions 204 comprise sidewalls205 with a first segment 205 a and a second segment 205 b overlying thefirst segment 205 a. The first segment 205 a has a linear surface (e.g.,along a crystalline plane) having a first sidewall angle ⊖₁ that is afirst acute angle measured with respect to a plane 203 extending alongbottoms of the plurality of protrusions 204. The second segment 205 bhas a second sidewall angle ⊖₂ that is a second acute angle measuredwith respect to the plane 203. The second sidewall angle ⊖₂ is largerthan the first sidewall angle ⊖₁. In some embodiments, the plurality ofprotrusions 204 may comprise cones, conical cylinders, or pyramids(e.g., having an n-sided base, wherein n=3, 4, 5, 6, . . . ). In someembodiments, the plurality of protrusions 204 may have sidewalls thatmeet at a pinnacle of the protrusions 204.

The plurality of protrusions 204 are separated from one another byrecesses 206 defined by the sidewalls 205. In some embodiments, thesidewalls 205 of adjacent ones of the plurality of protrusions 204 mayintersect at a lowest point within respective ones of the recesses 206.In other embodiments, the recesses 206 may respectively have a bottomsurface 208 arranged between the sidewalls 205. The bottom surface 208may comprise a substantially flat surface or a curved surface directlycoupled to a substantially linear surface of the first segment 205 a ofthe sidewalls 205. In some embodiments, the bottom surface 208 may havea width w that is in a range of between approximately 0 nm andapproximately 25 nm. In other embodiments, the bottom surface 208 has awidth w that is in a range of between approximately 0 nm andapproximately 15 nm. It has been appreciated that as a size of thebottom surface 208 decreases, the reflection of incident radiation 201a-201 b decreases for radiation in visible and near infrared (NIR)regions of the electromagnetic spectrum.

The plurality of protrusions 204 have a height 210 and a width 212(measured along largest dimensions of a protrusion). In someembodiments, an aspect ratio between the height 210 and the width 212 isin a range of between approximately 1 and approximately 1.25 (i.e.,1≤height/width≤1.25). In other embodiments, the aspect ratio may be in arange of between approximately 1 and approximately 1.2. In someembodiments, the height 210 may be in a range of between approximately400 nm and approximately 600 nm and the width 212 may be in a range ofbetween approximately 400 nm and approximately 500 nm. In otherembodiments, the height 210 and the width 212 may be less than 400 nm.

One or more absorption enhancement layers 222 are arranged over theplurality of protrusions 204 and within the recesses 206. In someembodiments, the one or more absorption enhancement layers 222 maycomprise a dielectric material (e.g., SiO₂). In other embodiments, theone or more absorption enhancement layers 222 may comprise a differentmaterial, such as a semiconductor material. The one or more absorptionenhancement layers 222 define an absorption enhancement structure 223comprising an interface having a topography that is configured toincrease absorption of radiation by the image sensing element 104.

In some embodiments, the absorption enhancement structure 223 isconfigured to increase absorption of radiation by providing for a lowreflection of radiation from the substrate 202 (e.g., a reflection ofless than or equal to approximately 5% for radiation having a wavelengthof between approximately 500 nm and approximately 700 nm). For example,for incident radiation 201 a having an angle of incidence α₁ greaterthan a critical angle, the angled sidewalls 205 may act to reflect theincident radiation 201 a to within the recesses 206, where the incidentradiation 201 a can be subsequently absorbed into the substrate 202. Theangled sidewalls 205 may further act to reduce an angle of incidence α₂for incident radiation 201 b having a steep angle with respect to a topsurface of the one or more absorption enhancement layers 222, therebypreventing the incident radiation 201 b from reflecting from thesubstrate 202.

As shown in the top-view 224, the plurality of protrusions 204 arearranged within the pixel region 103 in rows and columns. In someembodiments, the pixel region 103 may have a size 226 that is in a rangeof between approximately 0.9 μm and approximately 2.2 μm. In variousembodiments, the pixel region 103 may comprise different numbers of rowsand columns of protrusions 204. In some embodiments, the pixel region103 may comprise 4 rows and 4 columns of protrusions 204, so that thepixel region 103 comprises a 4×4 array of protrusions 204. It has beenappreciated that the 4×4 array of protrusions 204 provides the imagesensing element 104 within the pixel region 103 with a balanced QEbetween electromagnetic radiation within a visible region of theelectromagnetic spectrum (e.g., electromagnetic radiation havingwavelengths between approximately 400 nm and approximately 700 nm) andelectromagnetic radiation within a near infrared (NIR) region of theelectromagnetic spectrum (e.g., electromagnetic radiation havingwavelengths between approximately 700 nm and approximately 2000 nm). Forexample, a 4×4 array of protrusions 204 can provide for a QE ofapproximately 71% for green light and approximately 34% for NIRradiation at approximately 850 nm.

Referring again to cross-sectional view 200, the plurality ofprotrusions 204 are bordered by a ridge 216 arranged on opposing sidesof the pixel region 103. The ridge 216 comprises sidewalls having aplurality of different slopes and a substantially flat upper surface218. The ridge 216 has a height that extends over the plurality ofprotrusions 204 by a non-zero distance 220. In some embodiments, theridge 216 may have a height that is in a range of between approximately5% and approximately 15% larger than the plurality of protrusions 204.For example, the ridge 216 may have a height that is in a range ofbetween approximately 500 nm and approximately 600 nm.

As shown in the top-view 224, the ridge 216 continuously extends aroundan outer perimeter of the pixel region 103, so that the ridge 216surrounds the pixel region 103 and completely separates the pixel region103 from adjacent pixel regions.

FIG. 3 illustrates a cross-sectional view of some embodiments of aback-side illuminated CMOS image sensor (BSI-CIS) integrated chip 300comprising an absorption enhancement structure.

The BSI-CIS integrated chip 300 comprises a plurality of transistor gatestructures 304 arranged along a front-side 302 f of a substrate 302. Insome embodiments, the plurality of transistor gate structures 304 maycorrespond to a transfer transistor, a source-follower transistor, a rowselect transistor, and/or a reset transistor. The plurality oftransistor gate structures 304 have a gate dielectric layer 304 ddisposed along the front-side 302 f of the substrate 302 and a gateelectrode 304 e arranged on the gate dielectric layer 304 d. In someembodiments, sidewall spacers 304 s are arranged on opposing sides ofthe gate electrode 304 e.

In some embodiments, a transistor gate structure 304 corresponding to atransfer transistor is laterally arranged between a photodiode 306 and afloating diffusion well 308. In such embodiments, the photodiode 306 maycomprise a first region 305 within the substrate 302 having a firstdoping type (e.g., n-type doping) and an adjoining second region 307within the substrate 302 having a second doping type (e.g., p-typedoping) that is different than the first doping type. The transistorgate structure 304 is configured to control the transfer of charge fromthe photodiode 306 to the floating diffusion well 308. If a charge levelis sufficiently high within the floating diffusion well 308, asource-follower transistor (not shown) is activated and charges areselectively output according to operation of a row select transistor(not shown) used for addressing. A reset transistor (not shown) isconfigured to reset the photodiode 306 between exposure periods.

A dielectric structure 110 is arranged along the front-side 302 f of thesubstrate 302. The dielectric structure 110 may comprise a plurality ofstacked inter-level dielectric (ILD) layers. In various embodiments, theplurality of stacked inter-level dielectric (ILD) layers may compriseone or more of an oxide (e.g., SiO₂, SiCO, etc.), a fluorosilicateglass, a phosphate glass (e.g., borophosphate silicate glass), etc. Thedielectric structure 110 surrounds a plurality of conductiveinterconnect layers 112 electrically coupled to the transistor gatestructures 304. In some embodiments, the plurality of conductiveinterconnect layers 112 may comprise one or more of copper, aluminum,tungsten, and carbon nanotubes, for example. In some embodiments, thedielectric structure 110 is coupled to a carrier substrate 310configured to provide structural support to the BSI-CIS integrated chip300. In some embodiments, the carrier substrate 310 may comprisesilicon.

Pixel regions 103 a-103 c are laterally separated from one another by aplurality of isolation structures. In some embodiments, the plurality ofisolation structures may comprise a plurality of shallow trenchisolation (STI) structures 312 comprising one or more dielectricmaterials (e.g., SiO₂) arranged within trenches in the front-side 302 fof the substrate 302. In some embodiments, the plurality of isolationstructures may comprise a plurality of back-side deep trench isolation(BDTI) structures 314 comprising one or more dielectric materials (e.g.,SiO₂) arranged within trenches in a back-side 302 b of the substrate302. In some embodiments, the plurality of BDTI structures 314 may bearranged within a flat surface along the back-side 302 b of thesubstrate 302 between adjacent ones of the pixel regions 103 a-103 c(e.g., along a ridge 216, shown in FIG. 2A, having a height greater thanprotrusions 204). In some embodiments, the plurality of BDTI structures314 may respectively have a width that is smaller than a width of one ofthe plurality of STI structures 312. In some embodiments, the pluralityof isolation structures may comprise a deep-well region 316 and/or acell-well region 318 having one or more doping types that providefurther isolation between adjacent pixel regions 103 a-103 c by way ofjunction isolation. The deep-well region 316 is arranged in thesubstrate 302 at a location laterally aligned with the STI structure 312and/or the BDTI structure 314. The cell-well region 318 is arranged inthe substrate 302 at a location vertically between the deep-well region316 and the STI structure 312.

The back-side 302 b of the substrate 302 has a non-planar surfacecomprising a plurality of protrusions 204. One or more absorptionenhancement layers 320 are arranged along the non-planar back-side 302 bof the substrate 302. The one or more absorption enhancement layers 320are comprised within an absorption enhancement structure 321 configuredto improve absorption of radiation by the photodiode 306. In someembodiments, the one or more absorption enhancement layers 320 maycomprise an anti-reflective coating 320 a and a dielectric layer 320 bseparated from the back-side 302 b of the substrate 302 by theanti-reflective coating 320 a. In some embodiments, the anti-reflectivecoating 320 a may comprise a high-k dielectric material (e.g., hafniumoxide, nickel oxide, zirconium oxide etc.). In some embodiments, thedielectric layer 320 b may comprise an oxide (e.g., SiO₂).

In some embodiments, the absorption enhancement structure 321 maycomprise a nano-structure. For example, in some embodiments theabsorption enhancement structure 321 may comprise a silicon nano-pillararray (Si-NPA). In such embodiments, the substrate 302 may comprise aporous silicon layer (not shown) arranged over a crystalline siliconbulk and comprising the plurality of protrusions 204. In contrast tocrystalline silicon, which is an indirect band gap semiconductor, theSi-NPA is able to directly absorb photons (due to the quantumconfinement effect of the carriers in silicon nano-crystallites of theSi-NPA), thereby increasing absorption of radiation by the substrate302. In other embodiments, the absorption enhancement structure 321 maycomprise a two-dimensional photonic crystal configured to selectivelytransmit photons within a certain photonic energy band, while blockingthe transmission of photons outside of the photonic energy band. Thetwo-dimensional photonic crystal can be used to transmit photons to thesubstrate 302 and to block reemitted photons, thereby effectivelytrapping reemitted photons inside the substrate 302. The trappedreemitted photons are subsequently reabsorbed by the substrate 302,which increases absorption by the substrate 302.

In some embodiments, a dielectric planarization structure 322 may bearranged over the one or more absorption enhancement layers 320. Thedielectric planarization structure 322 has a substantially planar uppersurface 322 u. In various embodiments, the dielectric planarizationstructure 322 may comprise one or more stacked dielectric layers 322a-322 b. For example, in some embodiments, the dielectric planarizationstructure 322 may comprise a first dielectric layer 322 a comprising afirst material and a second dielectric layer 322 b stacked onto thefirst dielectric layer 322 a and comprising a second material. In someembodiments, the first material and/or the second material may comprisean oxide (e.g., SiO₂) or a nitride, for example.

A grid structure 324 is disposed over the dielectric planarizationstructure 322. The grid structure 324 comprises sidewalls that defineopenings overlying the pixel regions 103 a-103 c. In variousembodiments, the grid structure 324 may comprise a metal (e.g.,aluminum, cobalt, copper, silver, gold, tungsten, etc.) and/or adielectric material (e.g., SiO₂, SiN, etc.). A plurality of colorfilters, 326 a-326 c, are arranged within the openings in the gridstructure 324. The plurality of color filters, 326 a-326 c, arerespectively configured to transmit specific wavelengths of incidentradiation. For example, a first color filter 326 a may transmitradiation having wavelengths within a first range (e.g., correspondingto green light), while a second color filter 326 b may transmitradiation having wavelengths within a second range (e.g., correspondingto red light) different than the first range, etc. A plurality ofmicro-lenses 328 are arranged over the plurality of color filters 326a-326 c. Respective ones of the plurality of micro-lenses 328 arelaterally aligned with the color filters, 326 a-326 c, and overlie thepixel regions 103 a-103 c. The plurality of micro-lenses 328 areconfigured to focus the incident radiation (e.g., light) towards thepixel regions 103 a-103 c.

FIGS. 4-9 illustrate cross-sectional views of some embodiments of amethod of forming an absorption enhancement structure for an imagesensor integrated chip. Although the cross-sectional-views shown inFIGS. 4-9 are described with reference to a method of forming anabsorption enhancement structure for an image sensor integrated chip, itwill be appreciated that the structures shown in the figures are notlimited to the method of formation but rather may stand alone separateof the method.

As illustrated in cross-sectional view 400 of FIG. 4, a substrate 402 isprovided. The substrate 402 comprises a first side 402 a and a secondside 402 b. The substrate 402 may be any type of semiconductor body(e.g., silicon, SiGe, SOI, etc.), such as a semiconductor wafer and/orone or more die on a wafer, as well as any other type of semiconductorand/or epitaxial layers, associated therewith. In some embodiments, thesubstrate 402 may comprise a base substrate 404 and an epitaxial layer406. In some embodiments, the base substrate 404 and/or the epitaxiallayer 406 may comprise silicon. Alternatively, the base substrate 404and/or the epitaxial layer 406 may comprise silicon germanium, galliumarsenide, or another semiconductor material. In some embodiments, thesubstrate 402 may not have an epitaxial layer.

As shown in cross-sectional view 500 of FIG. 5, a patterned maskinglayer 502 is formed over the first side 402 a of the substrate 402. Thepatterned masking layer 502 comprises sidewalls defining openings 504arranged in a periodic pattern over the substrate 402. In someembodiments, the patterned masking layer 502 may be formed by depositinga layer of photosensitive material (e.g., a positive or negativephotoresist) over the substrate 402. The layer of photosensitivematerial is selectively exposed to electromagnetic radiation 508according to a photomask 506. The electromagnetic radiation 508 modifiesa solubility of exposed regions within the photosensitive material todefine soluble regions. The photosensitive material is subsequentlydeveloped to define the openings 504 within the photosensitive materialby removing the soluble regions.

As shown in cross-sectional view 600 of FIG. 6A, a dry etching processis performed on the first side 402 a of the substrate 402 according tothe patterned masking layer (502 of FIG. 5). The dry etching process isperformed by exposing unmasked regions of the substrate 402 to a dryetchant 602. The dry etchant 602 removes parts of the substrate 402 inthe unmasked regions to define a plurality of recesses arranged betweena plurality of intermediate protrusions 604 extending outward from thesubstrate 402. In some embodiments, the plurality of intermediateprotrusions 604 comprise tapered protrusions having one or more angledsidewalls 605. The angled sidewalls 605 respectively comprise a firstsegment 605 a having a first slope and a second segment 605 b overlyingthe first segment 605 a and having a second slope that is smaller thanthe first slope. In some embodiments, the plurality of intermediateprotrusions 604 may comprise a first intermediate protrusion 604 aseparated from a second intermediate protrusion 604 b by a bottomsurface 606 comprising a substantially horizontal bottom surface. Insome embodiments, the first segment 605 a may have a sidewall that iscoupled directly to the bottom surface 606 and that forms an acute angleα₁ of greater than 55° with respect to a plane extending along thebottom surface 606.

In some embodiments, the dry etching process may comprise a plasmaetching process. For example, the plasma etching process may comprise acoupled plasma etching process, such as an inductively coupled plasma(ICP) etching process or a capacitively coupled plasma (CCP) etchingprocess. In other embodiments, the dry etching process may comprise asputter etching process, an ion milling process, or a reactive ionetching (RIE) process. In some embodiments, the dry etchant 602 may havean etching chemistry comprising a fluorine species (e.g., CF₄, CHF₃,C₄F₈, etc.). The dry etching process may result in a damaged region 608arranged along outer surfaces of the plurality of intermediateprotrusions 604. The damaged region 608 includes defects (e.g.,interstitial and/or vacancies) within the crystalline lattice of thesubstrate 402, which can negatively impact performance of a resultingimage sensor integrated chip.

In some embodiments, wherein the absorption enhancement structurecomprises a silicon nano-pillar array (Si-NPA), a layer of poroussilicon (not shown) may be formed along the first side 402 a of thesubstrate 402 prior to the dry etch process. In some embodiments, thelayer of porous silicon may be formed by exposing the first side 402 aof the substrate 402 to hydroflouric acid. In some additionalembodiments, the layer of porous silicon may be formed by exposing thefirst side 402 a of the substrate 402 to a stain etching processcomprising hydroflouric acid, nitric acid, and water.

In some embodiments, the dry etchant 602 may remove the masking layer(502 of FIG. 5) from over the substrate 402. In other embodiments (notshown), the patterned masking layer (502 of FIG. 5) may be removed by aplasma stripping/ashing process performed after the dry etching processand prior to exposing the substrate 402 to a wet etchant (shown in FIG.6B). In yet other embodiments (not shown), the patterned masking layer(502 of FIG. 5) may be kept in place while exposing the substrate 402 toa wet etchant (shown in FIG. 6B) and subsequently removed during orafter the wet etching process.

As shown in cross-sectional view 610 of FIG. 6B, a wet etching processis performed on the first side 402 a of the substrate 402 after the dryetching process is finished. The wet etching process is performed byexposing the first side 402 a of the substrate 402 to a wet etchant 612.The wet etchant 612 removes the damaged region (608 of FIG. 6A) of theplurality of intermediate protrusions (604 of FIG. 6A). The wet etchingprocess also changes a profile of the plurality of intermediateprotrusions 604 to form a plurality of protrusions 114 respectivelyhaving sidewalls 115 comprising a first segment 115 a having a firstslope a second segment 115 b overlying the first segment 115 a andhaving a second slope that is larger than the first slope. In someembodiments, the first segment 115 a may have a sidewall angle ⊖₁ in arange of between approximately 45° and approximately 55°.

In some embodiments, the wet etchant 612 may comprisetetramethylammonium hydroxide (TMAH). In other embodiments, the wetetchant 612 may comprise potassium hydroxide (KOH) or a similar etchant.The wet etchant 612 may increase a depth of recesses 116 arrangedbetween the protrusions 114 from the first depth d₁ (shown in FIG. 6A)to a larger second depth d₂ (d₂>d₁).

The wet etchant 612 may also reduce a width of the bottom surface 606from a first width w₁ (shown in FIG. 6A) to a second width w₂ smallerthan the first width w₁. In some embodiments, the wet etchant 612changes the bottom surface from a substantially horizontal surface to acurved surface. Reducing a width of the bottom surface 606 reduces areflection of radiation from the substrate 402 in comparison to thestructure shown in FIG. 6A (formed by a dry etching process, but not awet etching process) by between approximately 3% and approximately 5%.This corresponds to a reduction in reflection of between approximately58% and approximately 60% relative to a substrate having a flat firstsurface.

As shown in cross-sectional view 700 of FIG. 7, one or more absorptionenhancement layers 702 are formed over and between the plurality ofprotrusions 114. In various embodiments, the one or more absorptionenhancement layers 702 may comprise a dielectric material (e.g., siliconoxide, TEOS, etc.). In some embodiments, the one or more absorptionenhancement layers 702 may comprise an anti-reflective coating 320 a anda dielectric layer 704 separated from the plurality of protrusions 114by the anti-reflective coating 320 a. In some embodiments, theanti-reflective coating 320 a may comprise a high-k material and thedielectric layer 704 may comprise an oxide. The one or more absorptionenhancement layers 702 may comprise an upper surface 702 u having aplurality of curved surfaces arranged over the protrusions 114 andintersecting one another.

As shown in cross-sectional view 800 of FIG. 8, a planarization process(e.g., a chemical mechanical planarization process) may be performedafter depositing the one or more absorption enhancement layers 320. Theplanarization process forms a substantially planar surface 320 u alongan upper surface of the one or more absorption enhancement layers 320.The substantially planar surface 320 u faces away from the substrate402.

FIG. 9 illustrates a flow diagram of some embodiments of a method 900 offorming an absorption enhancement structure for an image sensorintegrated chip.

While disclosed methods (e.g., methods 900, 1900, and 2700) areillustrated and described herein as a series of acts or events, it willbe appreciated that the illustrated ordering of such acts or events arenot to be interpreted in a limiting sense. For example, some acts mayoccur in different orders and/or concurrently with other acts or eventsapart from those illustrated and/or described herein. In addition, notall illustrated acts may be required to implement one or more aspects orembodiments of the description herein. Further, one or more of the actsdepicted herein may be carried out in one or more separate acts and/orphases.

At 902, a substrate is provided. FIG. 4 illustrates a cross-sectionalview 400 of some embodiments corresponding to act 902.

At 904, a patterned masking layer is formed over a first side of thesubstrate. FIG. 5 illustrates a cross-sectional view 500 of someembodiments corresponding to act 904.

At 906, a dry etching process is performed according to the patternedmasking layer to define a plurality of recesses and/or intermediateprotrusions within the first side of the substrate. FIG. 6A illustratesa cross-sectional view 600 of some embodiments corresponding to act 906.

At 908, the patterned masking layer may be removed, in some embodiments.FIG. 6B illustrates a cross-sectional view 610 of some embodimentscorresponding to act 908.

At 910, a wet etching process is performed on the first side of thesubstrate after the dry etching process is finished to define aplurality of protrusions from the plurality of intermediate protrusions.FIG. 6B illustrates a cross-sectional view 610 of some embodimentscorresponding to act 910.

At 912, one or more absorption enhancement layers are formed over andbetween the plurality of protrusions. In some embodiments, the one ormore absorption enhancement layers may be formed by depositing adielectric material over the first side of the substrate, at 914. Aplanarization process is subsequently performed to give the one or moreabsorption enhancement layers a planar surface facing away from thesubstrate, at 916. FIGS. 7-8 illustrate cross-sectional views, 700 and800, of some embodiments corresponding to act 912.

FIGS. 10-18 illustrate cross-sectional views of some embodiments of amethod of forming a BSI-CIS comprising an absorption enhancementstructure.

As illustrated in cross-sectional view 1000 of FIG. 10, a substrate 402is provided. In some embodiments, the substrate 402 may comprise a basesubstrate 404 and an epitaxial layer 406. The epitaxial layer 406comprises a front-side 406 f contacting the base substrate 404 and aback-side 406 b. In some such embodiments, the epitaxial layer 406and/or the base substrate 404 may comprise a semiconductor material,such as silicon. In other embodiments, the substrate 402 may notcomprise an epitaxial layer.

As shown in cross-sectional view 1100 of FIG. 11, a patterned maskinglayer 502 is formed along the back-side 406 b of the epitaxial layer406. The patterned masking layer 502 comprises sidewalls definingopenings 504 arranged in a periodic pattern over the epitaxial layer406.

As shown in cross-sectional view 1200 of FIG. 12A, a dry etching processis performed on the back-side 406 b of the epitaxial layer 406 accordingto the patterned masking layer (502 of FIG. 11). The dry etching processexposes unmasked regions of the back-side 406 b of the epitaxial layer406 to a dry etchant 602, which removes parts of the epitaxial layer 406to define a plurality of intermediate protrusions 604 arranged in aperiodic pattern along the back-side of the epitaxial layer 406. Theplurality of intermediate protrusions 604 are laterally separated fromone another by recesses within the back-side 406 b of the epitaxiallayer 406. In some embodiments, the plurality of intermediateprotrusions 604 may have a profile as described above in relation toFIG. 6A. The dry etching process may result in a damaged region 608arranged along outer edges of the plurality of intermediate protrusions604. The damaged region 608 includes defects (e.g., interstitial and/orvacancies) within the crystalline lattice of the epitaxial layer 406.

As shown in cross-sectional view 1202 of FIG. 12B, a wet etching processis performed on the back-side 406 b of the epitaxial layer 406 after thedry etching process is finished. The wet etching process exposes theback-side 406 b of the epitaxial layer 406 to a wet etchant 612 (e.g.,TMAH, KOH, etc.). The wet etchant 612 etches the back-side 406 b of theepitaxial layer 406 and removes the damaged region (608 of FIG. 12A) toform a plurality of protrusions 114 from the plurality of intermediateprotrusions (604 of FIG. 12A). The wet etching process causes theplurality of protrusions 114 to respectively have sidewalls 115comprising a first segment 115 a with a first slope and a second segment115 b overlying the first segment 115 a and having a second slope thatis larger than the first slope.

As shown in cross-sectional view 1300 of FIG. 13, one or more absorptionenhancement layers 320 are formed over and between the plurality ofprotrusions 114. The one or more absorption enhancement layers 320 maybe formed by depositing a dielectric material (e.g., silicon oxide,TEOS, etc.) onto the back-side 406 b of the epitaxial layer 406. Aplanarization process (e.g., a CMP process) may subsequently beperformed on the one or more absorption enhancement layers 320.

As shown in cross-sectional view 1400 of FIG. 14, the one or moreabsorption enhancement layers 320 are bonded to a support substrate1402. In some embodiments, the support substrate 1402 may comprise asilicon substrate. In some embodiments, the base substrate 404 may beremoved after bonding to thin the substrate and allow for radiation topass through the epitaxial layer 406 to image sensing elements (104 ofFIG. 15). In various embodiments, the substrate 402 may be removed byetching and/or mechanical grinding a front-side of the substrate 402. Inother embodiments, wherein the substrate 402 does not comprise anepitaxial layer, the substrate 402 may be thinned after bonding toreduce a thickness of the substrate 402. In various embodiments, thesubstrate 402 may be thinned and/or removed by etching and/or mechanicalgrinding a front-side of the substrate 402.

As shown in cross-sectional view 1500 of FIG. 15, image sensing elements104 are formed within pixel regions 103 a-103 b of the epitaxial layer406. In some embodiments, the image sensing elements 104 may comprisephotodiodes formed by implanting one or more dopant species into thefront-side 406 f of the epitaxial layer 406. For example, thephotodiodes may be formed by selectively performing a first implantationprocess (e.g., according to a masking layer) to form a first regionhaving a first doping type (e.g., n-type), and subsequently performing asecond implantation process to form a second region abutting the firstregion and having a second doping type (e.g., p-type) different than thefirst doping type. In some embodiments a floating diffusion well (notshown) may also be formed using one of the first or second implantationprocesses.

One or more transistor gate structures 304 are formed along thefront-side 406 f of the epitaxial layer 406 within the pixel regions 103a-103 b. In various embodiments, the one or more transistor gatestructures 304 may correspond to a transfer transistor, asource-follower transistor, a row select transistor, and a resettransistor. In some embodiments, the one or more transistor gatestructures 304 may be formed by depositing a gate dielectric film and agate electrode film on the front-side 406 f of the epitaxial layer 406.The gate dielectric film and the gate electrode film are subsequentlypatterned to form a gate dielectric layer 304 d and a gate electrode 304e. Sidewall spacers 304 s may be formed on the outer sidewalls of thegate electrode 304 e. In some embodiments, the sidewall spacers 304 smay be formed by depositing a spacer layer (e.g., a nitride, an oxide,etc.) onto the front-side 406 f of the epitaxial layer 406 andselectively etching the nitride to form the sidewall spacers 304 s.

In some embodiments, one or more shallow trench isolation (STI)structures 312 may be formed within the front-side 406 f of theepitaxial layer 406 on opposing sides of the pixel regions 103 a-103 b.The STI structures 312 may be formed by selectively etching thefront-side 406 f of the epitaxial layer 406 to form trenches andsubsequently forming one or more dielectric materials within thetrenches. In some embodiments, the STI structures 312 may be formedprior to formation of the one or more transistor gate structures 304,the image sensing elements 104 and/or the floating diffusion well.

As shown in cross-sectional view 1600 of FIG. 16, a plurality ofconductive interconnect layers 112 are formed within a dielectricstructure 110 formed along the front-side 406 f of the epitaxial layer406. In some embodiments, the plurality of conductive interconnectlayers 112 may be formed using a damascene process (e.g., a singledamascene process or a dual damascene process). The damascene process isperformed by forming an ILD layer over the front-side 406 f of theepitaxial layer 406, etching the ILD layer to form a via hole and/or ametal trench, and filling the via hole and/or metal trench with aconductive material. In some embodiments, the ILD layer may be depositedby a physical vapor deposition technique (e.g., PVD, CVD, PE-CVD, ALD,etc.) and the conductive material may be formed using a depositionprocess and/or a plating process (e.g., electroplating, electro-lessplating, etc.). In various embodiments, the plurality of conductiveinterconnect layers 112 may comprise tungsten, copper, or aluminumcopper, for example.

As shown in cross-sectional view 1700 of FIG. 17, the dielectricstructure 110 is bonded to a carrier substrate 310 and the supportsubstrate (1402 of FIG. 16) is subsequently removed. In someembodiments, the bonding process may use an intermediate bonding oxidelayer (not shown) arranged between the dielectric structure and thecarrier substrate 310. In some embodiments, the bonding process maycomprise a fusion bonding process. In some embodiments, the carriersubstrate 310 may comprise a silicon substrate.

As shown in cross-sectional view 1800 of FIG. 18, a plurality of colorfilters 326 a-326 b are formed over the one or more absorptionenhancement layers 320. In some embodiments, the plurality of colorfilters 326 a-326 b may be formed within openings in a grid structure324 overlying the one or more absorption enhancement layers 320. In someembodiments, the plurality of color filters 326 a-326 b may be formed byforming a color filter layer and patterning the color filter layer. Thecolor filter layer is formed of a material that allows for thetransmission of radiation (e.g., light) having a specific range ofwavelength, while blocking light of wavelengths outside of the specifiedrange.

A plurality of micro-lenses 328 are formed over the plurality of colorfilters 326 a-326 b. In some embodiments, the plurality of micro-lenses328 may be formed by depositing a micro-lens material above theplurality of color filters (e.g., by a spin-on method or a depositionprocess). A micro-lens template (not shown) having a curved uppersurface is patterned above the micro-lens material. In some embodiments,the micro-lens template may comprise a photoresist material exposedusing a distributing exposing light dose (e.g., for a negativephotoresist more light is exposed at a bottom of the curvature and lesslight is exposed at a top of the curvature), developed and baked to forma rounding shape. The plurality of micro-lenses 328 are then formed byselectively etching the micro-lens material according to the micro-lenstemplate.

FIG. 19 illustrates a flow diagram of some embodiments of a method 1900of forming a BSI-CIS comprising an absorption enhancement structure.

At 1902, a substrate is provided. FIG. 10 illustrates a cross-sectionalview 1000 of some embodiments corresponding to act 1902.

At 1904, a plurality of recesses and/or protrusions are formed within aback-side of the substrate. In some embodiments, the plurality ofrecesses and/or protrusions may be formed by forming a patterned maskinglayer over the back-side of the substrate at 1906. A dry etching processmay be performed with the patterned masking layer in place to form aplurality of intermediate protrusions, at 1908. A wet etching process issubsequently performed after the dry etching process is completed toform a plurality of protrusions from the plurality of intermediateprotrusions, at 1910. FIGS. 11-12B illustrates cross-sectional views ofsome embodiments corresponding to act 1904.

At 1912, one or more absorption enhancement layers are formed over theback-side of the substrate. FIG. 13 illustrates a cross-sectional view1300 of some embodiments corresponding to act 1912.

At 1914, the one or more absorption enhancement layers are coupled to asupport substrate. FIG. 14 illustrates a cross-sectional view 1400 ofsome embodiments corresponding to act 1914.

At 1916, the substrate is thinned to reduce a thickness of thesubstrate. FIG. 14 illustrates a cross-sectional view 1400 of someembodiments corresponding to act 1916.

At 1918, an image sensing element is formed within a pixel region of asubstrate. FIG. 15 illustrates a cross-sectional view 1500 of someembodiments corresponding to act 1918.

At 1920, one or more transistor gate structures for transistor devicesare formed along a front-side of the substrate. FIG. 15 illustrates across-sectional view 1500 of some embodiments corresponding to act 1920.

At 1922, a plurality of conductive interconnect layer are formed withina dielectric structure along the front-side of the substrate. FIG. 16illustrates a cross-sectional view 1600 of some embodimentscorresponding to act 1922.

At 1924, the dielectric structure is coupled to a carrier substrate andthe support substrate is removed. FIG. 17 illustrates a cross-sectionalview 1700 of some embodiments corresponding to act 1924.

At 1926, color filters and micro-lenses are formed over the one or moreabsorption enhancement layers. FIG. 18 illustrates a cross-sectionalview 1800 of some embodiments corresponding to act 1926.

FIGS. 20-26 illustrate cross-sectional views of some embodiments of analternative method of forming a BSI-CIS comprising an absorptionenhancement structure.

As illustrated in cross-sectional view 2000 of FIG. 20, a substrate 402is provided. In some embodiments, the substrate 402 may comprise a basesubstrate 404 and an epitaxial layer 406 comprising a front-side 406 fand a back-side 406 b. In some such embodiments, the epitaxial layer 406and/or the base substrate 404 may comprise silicon. In otherembodiments, the substrate 402 may not comprise an epitaxial layer.

As shown in cross-sectional view 2100 of FIG. 21, image sensing elements104 are formed within pixel regions 103 a-103 b of the epitaxial layer406. In some embodiments, the image sensing elements 104 may comprisephotodiodes formed by implanting one or more dopant species into thefront-side 406 f of the epitaxial layer 406. In some embodiments, one ormore shallow trench isolation (STI) structures 312 may be formed withinthe front-side 406 f of the epitaxial layer 406 on opposing sides of thepixel regions 103 a-103 b.

As shown in cross-sectional view 2200 of FIG. 22, one or more transistorgate structures 304 are formed along the front-side 406 f of theepitaxial layer 406 within the pixel regions 103 a-103 b. A plurality ofconductive interconnect layers 112 (e.g., copper, aluminum, and/ortungsten metal interconnect layers) are formed within a dielectricstructure 110 formed along the front-side 406 f of the epitaxial layer406. In some embodiments, the plurality of conductive interconnectlayers 112 may be formed using a damascene process (e.g., a dualdamascene process).

As shown in cross-sectional view 2300 of FIG. 23, the dielectricstructure 110 is bonded to a carrier substrate 310.

As shown in cross-sectional view 2400 of FIG. 24, a patterned maskinglayer 502 is formed along the back-side 406 b of the epitaxial layer406. The patterned masking layer 502 comprises sidewalls definingopenings 504 arranged in a periodic pattern over the epitaxial layer406.

As shown in cross-sectional view 2500 of FIG. 25A, a dry etching processis performed on a back-side 406 b of the epitaxial layer 406 accordingto the patterned masking layer (502 of FIG. 24). The dry etching processexposes unmasked regions of the back-side 406 b of the epitaxial layer406 to a dry etchant 602, which removes parts of the epitaxial layer 406to define a plurality of intermediate protrusions 604 arranged in aperiodic pattern along the back-side of the epitaxial layer 406. In someembodiments, the plurality of intermediate protrusions 604 may have aprofile as described above in relation to FIG. 6A. The dry etchingprocess may result in a damaged region 608 arranged along outer edges ofthe plurality of intermediate protrusions 604 and having defects withinthe crystalline lattice of the epitaxial layer 406.

As shown in cross-sectional view 2502 of FIG. 25B, a wet etching processis performed on the back-side 406 b of the epitaxial layer 406 after thedry etching process is finished. The wet etching process exposes theback-side 406 b of the epitaxial layer 406 to a wet etchant 612 (e.g.,TMAH, KOH, etc.) to remove the damaged region (608 of FIG. 25A) to forma plurality of protrusions 114 from the plurality of intermediateprotrusions (604 of FIG. 25A). The wet etching process causes theplurality of protrusions 114 to respectively have sidewalls 115comprising a first segment 115 a having a first sidewall angle and asecond segment 115 b overlying the first segment 115 a and having asecond sidewall angle that is larger than the first sidewall angle.

As shown in cross-sectional view 2600 of FIG. 26, one or more absorptionenhancement layers 320 are formed over and between the plurality ofprotrusions 114. The one or more absorption enhancement layers 320 maybe formed by depositing a dielectric material (e.g., silicon oxide,TEOS, etc.) onto the back-side 406 b of the epitaxial layer 406. Aplurality of color filters 326 a-326 b are formed over the one or moreabsorption enhancement layers 320, and a plurality of micro-lenses 328are formed over the plurality of color filters 326 a-326 b. Aplanarization process (e.g., a CMP process) may subsequently beperformed on the one or more absorption enhancement layers 320.

FIG. 27 illustrates a flow diagram of some embodiments of an alternativemethod 2700 of forming a BSI-CIS comprising an absorption enhancementstructure.

At 2702, a substrate is provided. FIG. 20 illustrates a cross-sectionalview 2000 of some embodiments corresponding to act 2702.

At 2704, an image sensing element is formed within a pixel region of asubstrate. FIG. 21 illustrates a cross-sectional view 2100 of someembodiments corresponding to act 2704.

At 2706, one or more transistor gate structures for transistor devicesare formed along the front-side of the substrate. FIG. 22 illustrates across-sectional view 2200 of some embodiments corresponding to act 2706.

At 2708, a plurality of conductive interconnect layer are formed withina dielectric structure arranged along the front-side of the substrate.FIG. 22 illustrates a cross-sectional view 2200 of some embodimentscorresponding to act 2708.

At 2710, the substrate is thinned. In some embodiments, the dielectricstructure is coupled to a carrier substrate prior to thinning. FIG. 23illustrates a cross-sectional view 2300 of some embodimentscorresponding to act 2710.

At 2712, a plurality of recesses and/or protrusions are formed within aback-side of the substrate at a location overlying the image sensingelement. In some embodiments, the plurality of recesses and/orprotrusions may be formed by forming a patterned masking layer over theback-side of the substrate at 2714. A dry etching process may beperformed with the masking layer in place to form a plurality ofintermediate protrusions, at 2716. A wet etching process is subsequentlyperformed after the dry etching process is completed to form a pluralityof protrusions from the plurality of intermediate protrusions, at 2718.FIGS. 24-25B illustrates cross-sectional views of some embodimentscorresponding to act 2712.

At 2720, one or more absorption enhancement layers are formed over theback-side of the substrate. FIG. 26 illustrates a cross-sectional view2600 of some embodiments corresponding to act 2720.

At 2722, color filters and micro-lenses are formed over the one or moreabsorption enhancement layers. FIG. 26 illustrates a cross-sectionalview 2600 of some embodiments corresponding to act 2722.

Although the disclosed absorption enhancement structure has beendescribed with respect to back-side image sensors, it will beappreciated that the disclosed absorption enhancement structure is notlimited to such image sensors. For example, in various embodiments, thedisclosed absorption enhancement structure may be used in back-sideimage sensors or in front-side image sensors.

The present disclosure relates to a method of forming an absorptionenhancement structure that improves a quantum efficiency (QE) of anintegrated chip image sensor, and an associated apparatus. The methoduses a dry etching process and a wet etching process to reducecrystalline defects resulting from the formation of the absorptionenhancement structure.

In some embodiments, the present disclosure relates to a method offorming an absorption enhancement structure for an image sensorintegrated chip. The method comprises forming a patterned masking layerover a first side of a substrate, and performing a dry etching processon the first side of the substrate according to the patterned maskinglayer to define a plurality of intermediate protrusions arranged alongthe first side of the substrate. The method further comprises performinga wet etching process on the plurality of intermediate protrusions toform a plurality of protrusions, and forming one or more absorptionenhancement layers over and between the plurality of protrusions.

In other embodiments, the present disclosure relates to a method offorming an image sensor. The method comprises forming an image sensingelement within a substrate, and forming a patterned masking layer on afirst side of the substrate at a location over the image sensingelement. The method further comprises performing a dry etching processon the first side of the substrate according to the patterned maskinglayer to define a plurality of intermediate protrusions. The methodfurther comprises performing a wet etching process on the plurality ofintermediate protrusions to form a plurality of protrusions. The methodfurther comprises forming a dielectric material over the plurality ofprotrusions, and performing a planarization process on the dielectricmaterial.

In yet other embodiments, the present disclosure relates an image sensorintegrated chip. The image sensor integrated chip comprises an imagesensing element arranged within a substrate, and a plurality ofprotrusions arranged along a first side of the substrate over the imagesensing element. The image sensor integrated chip further comprises oneor more absorption enhancement layers arranged over and between theplurality of protrusions. The plurality of protrusions respectivelycomprise a sidewall having a first segment having a first sidewall angleand a second segment overlying the first segment and having a secondsidewall angle that is larger than the first sidewall angle. The firstsidewall angle and the second sidewall angle are acute angles measuredwith respect to a plane extending along bottoms of the plurality ofprotrusions.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of forming an absorption enhancementstructure for an image sensor integrated chip, comprising: forming apatterned masking layer over a first side of a substrate; performing adry etching process on the first side of the substrate according to thepatterned masking layer to define a plurality of intermediateprotrusions arranged along the first side of the substrate; performing awet etching process on the plurality of intermediate protrusions to forma plurality of protrusions; and forming one or more absorptionenhancement layers over and between the plurality of protrusions.
 2. Themethod of claim 1, wherein the wet etching process causes the pluralityof protrusions to respectively have a sidewall comprising a firstsegment having a linear surface with a first sidewall angle; and whereinthe first sidewall angle is a first acute angle measured with respect toa plane parallel to a second side of the substrate opposite the firstside.
 3. The method of claim 2, wherein the first sidewall angle is in arange of between approximately 45° and approximately 55°.
 4. The methodof claim 2, wherein the wet etching process causes the sidewall to havea second segment overlying the first segment and having a secondsidewall angle that is larger than the first sidewall angle; and whereinthe second sidewall angle is a second acute angle measured with respectto the plane extending along bottoms of the plurality of protrusions. 5.The method of claim 1, wherein the dry etching process forms a damagedregion having crystalline defects along outer edges of the plurality ofintermediate protrusions and the wet etching process removes the damagedregion.
 6. The method of claim 1, wherein forming the patterned maskinglayer comprises: forming an un-patterned masking layer over thesubstrate; and selectively removing parts of the un-patterned maskinglayer to form the patterned masking layer.
 7. The method of claim 1,further comprising: removing the patterned masking layer afterperforming the dry etching process and prior to performing the wetetching process.
 8. The method of claim 1, wherein the dry etchingprocess is configured to form a first intermediate protrusion separatedfrom a second intermediate protrusion by a bottom surface arrangedtherebetween; and wherein the wet etching process reduces a width of thebottom surface extending between a first protrusion formed from thefirst intermediate protrusion and a second protrusion formed from thesecond intermediate protrusion.
 9. The method of claim 1, wherein thedry etching process is configured to form a first intermediateprotrusion separated from a second intermediate protrusion by ahorizontal bottom surface arranged therebetween; and wherein the wetetching process changes the horizontal bottom surface to a curved bottomsurface.
 10. The method of claim 1, wherein the dry etching process isconfigured to form a first intermediate protrusion separated from asecond intermediate protrusion by a horizontal bottom surface arrangedtherebetween; and wherein the first intermediate protrusion has asidewall with a sidewall angle that is an acute angle of greater than55° with respect to a plane extending along the horizontal bottomsurface.
 11. A method of forming an image sensor, comprising: forming animage sensing element within a substrate; forming a patterned maskinglayer on a first side of the substrate at a location over the imagesensing element; performing a dry etching process on the first side ofthe substrate according to the patterned masking layer to define aplurality of intermediate protrusions; performing a wet etching processon the plurality of intermediate protrusions to form a plurality ofprotrusions; forming a dielectric material over the plurality ofprotrusions; and performing a planarization process on the dielectricmaterial.
 12. The method of claim 11, wherein the wet etching processcauses the plurality of protrusions to respectively have a sidewallcomprising a linear surface with a first sidewall angle that is an acuteangle in a range of between approximately 45° and approximately 55°measured with respect to a plane extending along bottoms of theplurality of protrusions.
 13. The method of claim 11, wherein the wetetching process causes the plurality of protrusions to respectively havea sidewall comprising a first segment with a first sidewall angle and asecond segment overlying the first segment and with a second sidewallangle that is larger than the first sidewall angle; and wherein thefirst sidewall angle and the second sidewall angle are acute anglesmeasured with respect to a plane extending along bottoms of theplurality of protrusions.
 14. The method of claim 11, wherein the dryetching process forms crystalline defects along outer edges of theplurality of intermediate protrusions and the wet etching processremoves the crystalline defects.
 15. The method of claim 11, wherein thedry etching process and the wet etching process etch silicon surfaces ofthe substrate.
 16. The method of claim 11, wherein the dry etchingprocess is configured to form a first intermediate protrusion separatedfrom a second intermediate protrusion by a bottom surface arrangedtherebetween; and wherein the wet etching process reduces a width of thebottom surface extending between a first protrusion formed from thefirst intermediate protrusion and a second protrusion formed from thesecond intermediate protrusion.
 17. The method of claim 11, wherein thepatterned masking layer comprises a photoresist layer.
 18. A method offorming an image sensor integrated chip, comprising: forming an imagesensing element within a pixel region of a semiconductor substrate;forming a plurality of conductive interconnect layers within adielectric structure disposed along a front side of the semiconductorsubstrate; performing a dry etching process on a backside of thesemiconductor substrate to define a plurality of intermediateprotrusions along the backside of the semiconductor substrate; andperforming a wet etching process on the plurality of intermediateprotrusions to form a plurality of protrusions along the backside of thesemiconductor substrate.
 19. The method of claim 18, wherein theplurality of protrusions respectively comprise a sidewall having a firstsegment with a first sidewall angle and a second segment overlying thefirst segment and having a second sidewall angle that is larger than thefirst sidewall angle; and wherein the first sidewall angle and thesecond sidewall angle are acute angles measured with respect to a planeextending along bottoms of the plurality of protrusions.
 20. The methodof claim 18, further comprising: forming a ridge along the backside ofthe semiconductor substrate surrounding the pixel region, wherein ahorizontal plane extending along tops of the plurality of protrusionsintersects sidewalls of the ridge.